Use StringBuilder instead of StringBuffer

[ivancea]

Replace non-multithreaded uses of StringBuffer to StringBuilder.

From the StringBuffer javadoc:

As of release JDK 5, this class has been supplemented with an equivalent class designed for use by a single thread, StringBuilder. The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization.

Just in case, and out of curiosity, made a little microbenchmark to check the difference both between a StringBuffer and a StringBuilder, and between a synchronized and non-synchronized method. The results are "as expected", and tell me that this won't affect negatively:

Benchmark                    (type)  (useSynchronized)   Score  Error  Units
TestsBenchmark.run   empty_function              false   0.089  0.004  ns/op
TestsBenchmark.run   empty_function               true   5.777  0.109  ns/op
TestsBenchmark.run  string_building              false  10.191  0.476  ns/op
TestsBenchmark.run  string_building               true  28.733  1.210  ns/op

The main reason to do this isn't performance anyway, but to avoid using this class when it's not needed.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[ivancea]

(Failing test is unrelated)

[craigtaverner]

Wow! I thought this had been done ages ago. I'm a little surprised to see these left-over bits.

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.19
✅	9.0